1. Field of the Invention
This invention relates to a fixed-focal-length lens system which is suitable for use as a projector lens in an enlarger or the like.
2. Description of the Prior Art
In a photographic enlarger for printing images borne by a negative film on a photographic paper, there has been used an orthometer type fixed-focal-length lens system due to easiness in correction of aberrations such as distortion.
However the known orthometer type fixed-focal-length lens system is disadvantageous in that the back focus (the distance between the negative film and the lens surface nearest to the negative film) is short and a mirror which is positioned between the lens system and the negative film to lead light transmitted through the negative film to a light measuring circuit for color correction makes it difficult for the operator to view the negative film when printing images borne by the negative film on a photographic paper.
Though, in a retro-focus type fixed-focal-length lens system, the back focus can be relatively long as compared with its focal length as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 64(1989)-61714 and 61(1986)-188512, the retro-focus type fixed-focal-length lens system is short in its focal length itself (10 mm or so) and accordingly its back focus cannot be sufficiently long. Further the retro-focus type fixed-focal-length lens system is disadvantageous in that it is difficult to correct aberrations when used at high magnifications due to an asymmetric arrangement with respect to the stop.